Introduction to Printed Circuit Board Design for EMC ...2014.psessymposium.org/sites/2014.psessymposium... · Signal Integrity Concerns It only takes one item listed below to cause

© 1996/2000. Montrose, M.I., Printed Circuit Board Design Techniques for EMC Compliance. Wiley/IEEE Press.© 1999. Montrose, M.I., EMC and the Printed Circuit Board - Design Theory and Layout Made Simple. Wiley/IEEE Press.

Introduction to Printed Circuit Board Design

For EMC Compliance

Mark MontrosePrinciple Consultant

Montrose Compliance Services, Inc.+ 1 (408) 247-5715

[email protected]

1


Fundamentals of Signal Integrity

2


What is Signal Integrity

The ability of electrical signals to travel from a source to load through a dielectric without loss of signal amplitude or parametric values

In order to solve a signal integrity problem, one must first understand transmission lines and how they function both in theory and reality

There are two kinds of design engineers:“Those that have signal integrity problems, and those that will”

3

Signal Integrity Concerns

It only takes one item listed below to cause a signal integrity problem

• Incorrect transmission line routing• Improper terminations• Power and/or return plane bounce• Improper RF return current path• Mode conversion• Rise time degradation• Lossy transmission lines at higher

frequencies• Poor power distribution network• Hidden parasitics (RLC)• Propagation delays• Skin depth and dielectric loss• Non-monotonic edges• Excessive inductance in the

transmission line• Excessive ringing and reflection• Lossy transmission lines• Poor printed circuit board material

• Overshoot and undershoot• Impedance discontinuities• Delta I noise• RC delay• Crosstalk• Stubs and their lengths• Excessive capacitive loaded lines• IR drops• Via stubs• Excess attenuation• Non-monotonic edges• Signal skew• Gaps in planes• Dispersion• Impedance discontinuities• Unknown parasitic• Intra line skew• High dielectric losses


4


Aspects of High-Speed Problems

Four aspects of high-speed problems are present in system designs

1. Signal quality: Reflections and distortions from impedance discontinuities in the signal or return path can affect the quality of the signal. A transmitted signal should see the same impedance throughout all interconnects (includes vias and connectors).

2. Crosstalk between nets: Mutual capacitance and inductance exists, both within an ideal and non-ideal return path. One must keep spacing of traces greater than a minimal value while minimizing mutual inductance, keeping the return path impedance as low as possible.

3. Rail collapse: A voltage drop within the power and return system when digital components switch logic states. One must minimize the impedance of the power and return path along with the delta-I (current).

4. EMI: Can be developed as a result of poor signal integrity within a transmission line. Must minimize bandwidth, ground impedance and common-mode coupling.

Details on impedance discontinuities, transmission line routing, terminations, and simulations are presented later.

5


totaltotalpdo

oo CLt

I(x)V(x) =

CL = Z

Lossless Transmission Line Equivalent Circuit Within a PCB

6


Lossy Transmission Line Equivalent Circuit Within a PCB

CjGLjRj LLLL

CjGLjRZo

LL

LL

characteristic impedanceZo

lengthlineLmay vary with frequencyG,R LL

jtxVxV o )exp()exp(),(

7


Lossy Transmission Lines

1. Resistive losses - Constant with frequency. Attenuation, usually measured in dB/unit distance, is proportional to the resistance per unit length of the conductor.

2. Skin effect losses - Proportional to the square root of frequency. As signal frequency increases, current flow retreats to the surface of the conductor flowing in a "skin" which becomes thinner with increasing frequency. Resistivity of the material stays the same, it is the cross section that decreases related to AC current flow.

3. Dielectric losses - The PCB material (core and prepreg) absorbs some of the electric field energy, which is directly proportional to frequency. Dielectric loss or dissipation factor (magnitude of energy loss) is not the same as dielectric constant (speed of signal travel).

4. Resonances - Typically caused by improperly terminated traces and split planes in addition to the lumped magnitude of both capacitance and inductance within power distribution networks.

8


Typical Transmission Line System

• Minimum reflections will occur when Zout = Zo and Zo= Zload• Maximum energy transfer occurs when Zout= Zo= Zload

If the load is not matched, a voltage is reflected back toward the source. The value of reflected voltage (Vr) and the percentage of the propagation signal reflected back towards the source (%) is:

where Vr = reflected voltageVo = source voltageRL = load resistanceZo = characteristic impedance of the transmission path

When RL is less than Zo, a negative reflected wave exist. If RL is greater than Zo, a positive wave is observed. This reflected wave will bounce back and forth between source and driver until dielectric losses absorbs the signal.

Zo

V Source

Zout

RLoad

VloadVsource

oLoad

oLoad or

Z+RZ-RV = V 100

Z+ZZ-Z =reflection %

oL

oL

RG174 - 50 RG58 - 50 RG59 - 75 RG62 - 93 TV Antenna - 300 Cable TV - 75 Twisted pairs - 70-120

9


Reflections – Poor Signal Integrity

Reflections will occur within a transmission line if not properly terminated. The following causes reflections.• Changes in trace width• Improperly matched termination networks• Lack of terminations• T-stubs, branched or bifurcated traces• Varying loads and logic families• Connector transitions• Any changes in impedance of the transmission line routing

Ringing indicates Rounding indicatesexcessive capacitancereflections (excessive inductance)

10


Crosstalk – Fundamental Aspects of Coupling

Zv = Zs (v)x ZL (v)Zs (v) ZL (v)+

Source Trace

Victim Trace

Victim Trace

Source TraceLm

Csv

CsvCvg

= Capacitance between source trace and victim trace= Capacitance between victim trace and ground

Csg = Capacitance between source trace and ground

Source Trace

Victim TraceCsv

Cvg

Z L (source)

Z L (victim)~

Zs (source)

VsZs (victim)

Inductive coupling

Csg

Ground plane or reference structureLm

Csv

AC

CA

B

B

D

D

Lm

11


Vs

Rs Ls

Rv Lv

Zg

Csg

Cvg

Z ZL1

L2Z

s1

Vv

Zs2

G (0V)

Schematic representation of a three wire circuit

MsvCsv

CsvCvg

= Capacitance between source trace and victim trace= Capacitance between victim trace and ground

Csg = Capacitance between source trace and ground

ZL1 ZL2

Csv

Msv

Vs

Zs1

Vv

Z s2

G (0V)

t

VsSource signal

t

Vv

Crosstalk onvictim trace

CvgCsg

Zg

Vs VvZs1 Zs2

ZL1 ZL2

Ground plane

Dielectric

Parallel traces over a ground plane

G (0V)

material

12

13

Stuck low

Coupled lines

Agressor line

Victim line

0.000 4.000 8.000 12.000 16.000 20.000Time (ns)

Voltage -V-

-3.000

-2.000

-1.000

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

Agressor line (source) - clock stimulus

Agressor line (load)

Victim line (source) - stuck lowVictim line (load)

0.010"

0.010"

0.010"

Signal

Power

Ground

Signal

0.008 0.008 0.0088.0 inches (20.3 cm) long, 72.1 ohmsPropagational delay: 1.126 nsOscillator: 66 MHz, 49% duty cycleCMOS, 3.3V, FastTraces: 0.008" wide and 0.008" apart (.20mm)Distance to reference plane: 0.010" (.25mm)



Far End Cross TalkOnly appears in surface traces and scales with coupling length, inversely with rise time and depends on line to line spacing. Reduce it by shorter coupling lengths, longer rise time, larger spacing, or best of all, route in stripline.

Near End Cross TalkSaturates with coupling length when coupling TD > ½ RT (rise time), predominantly affected by line to line spacing and can be reduced with lower dielectric constant..

Common Forms of Crosstalk

14


Sample List of Design Techniques to Prevent Crosstalk

To prevent crosstalk within a PCB, the following design and layout techniques are useful.• Crosstalk will increase with a wider trace width as mutual capacitance, Cm, increases.• With long parallel traces, greater mutual inductance, Lm, is present. • Crosstalk also increases with faster edge rates and frequency of operation.

1. Group logic devices according to functionality. 2. Minimize routed distance between components.3. Minimize parallel routed trace lengths.4. Locate components away from I/O interconnects and areas susceptible to field

corruption.5. Provide terminations for traces rich in harmonic energy.6. Avoid routing of traces parallel to each other with adequate separation between the

tracks.7. Route adjacent signal layers (either microstrip or stripline) orthogonally to prevents

capacitive and inductive coupling between two planes in parallel.8. Reduce signal-to-ground reference distance separation.9. Reduce trace impedance and/or signal drive level.10. Isolate signal layers routed in the same axis by a solid planar (typical of backplane

designs).

15


Power and/or Return Plane Bounce

dtdIL = V discharge

rnpower/returnpower/retu

16


Fundamentals of EMC

17


Component Characteristics at RF Frequencies(The Hidden Schematic)

18


How RF Energy is Created – Maxwell Made Simple

Maxwell’s four equations describe the relationship of electric and magnetic fields. Equations are derived from:

• Ampere’s Law• Faraday’s Law• Two from Gauss’s Law.

To overly simplify Maxwell, use Ohms law

Ohms Law (time domain-DC currents)V = I R

Ohms Law (frequency domain-AC currents)Vrf = Irf Z

19


Right Hand Rule(Faraday’s Law)

Field or Flux Line

I (Current in the wire)

(Magnetic Flux)

20


Maxwell's Equations(For Reference Purposes)

totals

s

s

vs

I dst D JdlH

t D + J = H

Ampere) (from Current Electric :Law Fourth

dst B - = dlE

t B - = E

Faraday) (from Potential Electric :Law Third

0 dsB 0 B

Gauss) (from Flux Magnetic :Law Second

0 dv dsD = D

Gauss) (from Flux Electric :Law First

m

e

21

Electric and Magnetic Field Impedance

A plane wave is a combination of both electric and magnetic field components (Poynting vector). Fields propagate from a field source near the velocity of light.

where o = 410-7 H/m o = 8.8510-12 F/m

Electric field component is measured in volts/meter (Note-voltage)Magnetic field component is in amps/meter (Note-current)

The ratio of both electric field (E) to magnetic field (H) is identified as the "impedance" of free space. This impedance ratio is described by:

Energy carried in the wave front is measured in Watts/meter2 (Note-power)


8oo 10 x3 = 1/=c

ohms 377 = εμ = HE =Z

o

o o

22

(Note-resistance)


23

d1H

dE

1

3d1H

dE 2

1

2d1H

dE 3

1


Electric and Magnetic Field Representation

Dipole Antenna Loop Antenna

24


Closed Loop Circuit

If a continuous, conductive low impedance RF return path is not present(transmission line impedance is greater than 377 ohms), the return path will befree space (377 at /4), which may be much less than the impedance of alocalized RF return path.

Signal path

Return path

Switch

25


Radiated Emissions from a Closed Loop Circuit

Loop Area

Radiated energy

Return current

Signal path

26

Loop Area Between Circuits or Components – Different Layers


27


Return Current Path of Travel – Multilayer Assembly

High frequency operationLow frequency operation

Illustration provided courtesy: Dr. Howard Johnson

28

Circuit Return Current Simulation @ F=1 Hz Circuit Return Current Simulation @ F=100 kHz

Circuit Return Current Simulation @ F=5 GHz

Courtesy of Alexander Perez, Agilent Technologies


29


Common-Mode and Differential-Mode Currents

Differential-mode1. Conveys desired information.2. Does not cause interference as the fields generated oppose each other and cancel out.

Common-mode1. The major source of cable radiation.2. Contains no useful information.3. Has no useful purpose.4. Causes a system (traces, cables, etc.) to radiate as a monopole antenna.

I1

E~

I2

Z~

Noise sourcein load

I2

Common-mode currentI = I 1 + I2total

Differential-mode currentI = I 1 - I2total

E~ Z

Noise sourcein loadI1

~+ +

I2'

2 2

½ CM½ DM

½ CM½ DM

30


Summary on How EMI Is Developed Within the PCB

1. Current transients are created by the production of high frequency periodicsignals injected into the power and return distribution network.

2. An RF voltage drop develops across any impedance within a transmissionline.

3. Common-mode RF currents are created by this RF voltage drop onunbalanced RF current return path.

4. Radiated emissions, created by these common-mode RF currents, areobserved on internal antenna structures by virtue of poor RF ground loop orreturn path control.

5. When any time-variant current is injected into a trace, magnetic flux isdeveloped, which in turn creates an electric field. The combination ofelectric and magnetic fields create a propagating plane wave.

6. Lack of a proper RF current return path exacerbates EMI.

31


Basic EMC Suppression and Grounding Concepts

32


Different Types of Grounds

• Signal Ground• Common Ground• Analog Ground• Digital Ground• Safety Ground• Noisy Ground• Quiet Ground• Earth Ground• Hardware Ground• Single-point Ground• Multi-point Ground• Shield Ground

What about: RF Ground?

Often a ground reference may serve multiple needs, each with a different application.

33

Defining “Ground”

Power/safety ground• Intended (neutral) and unintended (safety ground, generally the green wire)• 50/60/400 Hz

Lightning ground• A controlled path for lightning to reach the earth through a rod or metallic

structure• Generally a 1 MHz event and up to 100 kAmps per millisecond• Requires a high quality low ground resistance and inductance

Circuit/signal ground• Provides a return path for intended signal flow and for AC/DC power return;

mA to Amps• Requires a minimum low impedance path• Generally implemented as a ground plane or grids within a printed circuit

board


34

Defining “Ground” (continued)

EMI ground• Provides a controlled path for RF currents; DC to daylight, A to Amps• Requires a minimum ground impedance

ESD ground• Provides a controlled path for ESD currents • 0.7-3 ns rise times, 100-300 MHz, 10-50 Amps

RF ground• Provides an RF return path for flux to return to its source• Covers the entire frequency spectrum• Requires minimum impedance for maximum current/flux flow



35

36

• A ground system topology is determined by– Signal characteristics– System dimensions– System-specific separation and isolation requirements– Electrical safety requirements

• Primary ground system topologies includes– A “floating” system– Single-point (“star) ground (SPG)– Multi-point ground– Hybrid ground

Grounding System Topologies



Three Primary Grounding Methodologies

Single-Point Grounding: Series and Parallel Multi-Point

37


Single-Point: Best for use when signals are below 1 MHz.• Most sensitive circuit returns should be connected closest to the final

equipotential point.• Provides for greatest amount of loop currents to flow.• May be used between 1 and 10 MHz if longest conductor is < /20 of a

wavelength of highest frequency generated in the system.• Divided into two type: series and parallel.

Multi-Point: Preferred for frequencies above 1 MHz.• Minimized loop currents and ground impedance of the planes. A good low

inductance ground is necessary for high-frequency digital logic circuits. Ground plane(s) provides a low inductance ground return for RF currents.

• Lead inductance must be kept extremely short.• Provides for maximum EMI suppression at the PCB level.

Hybrid: For mixed technology products.• A combination of both single-point and multi-point grounding in the same

system

38


Resonance in a Multi-Point Ground

Printed circuit board.

Mounting plate or chassisMounting posts

Eddy currents

Internal power plane capacitance

Inductance in the power planes

ZZ I cm

Vcmby eddy currentsacross impedance(Z)post.

LC resonance in mounting posts

from mounting

produced

APPLICATION MODEL OF MULTIPOINT GROUNDING

ZtZ t cm2Vcm2V

cmI

ZB

Vcm1

Chassis

ELECTROMAGNETIC MODEL OF MULTI-POINT GROUNDING

V cm2 is reduced by the mounting posts (ground stitch locations).Resonance is thus controlled, along with enhanced RF suppression.

39


RF Current Density Distribution

where I(d) = signal current density, (A/inch or A/cm)Io = total current (A)H = height of the trace above the ground plane (in. or cm)D = perpendicular distance from the center line of the trace (in. or cm)

21

1

HDH

I = dI o

)(

40


Ground Slots Created with Through-Hole Components

41


Functional Partitioning

Slow speed I/O

CPU and clock logic

interconnects

Memory

Video Audio

Memory section

Power supply

Adapter cardsSupport logic

Analog processing

42


Bypassing and Decoupling(Power Distribution Networks)

43


The Need for Optimal Power Distribution

• Provides a stable voltage reference between components to ensure functional operation

• Distribute optimal power to all logic devices to minimize planar bounce

Key items of concernUse low impedance connections between logic gates:• The impedance between power pins on gates should be just as low as the

impedance between the return pins on the same device• A low impedance path must always be provided between power and return

Input+

-

+

-

+

-

Load C

Inductance in the Power Path

Charging current for capacitor C flows throughthe inductance of the power distribution network

I

Signal

Inductance in the Return Path

44


Power Distribution Networks as Transmission Lines

Power distribution networks can be represented as a two conductor transmission line with a defined characteristic impedance and propagation delay.

45


Primary Requirements for Enhanced Power Distribution

The amplitude of power supply transients are directly proportional to the characteristic impedance of the power distribution system, Z0:

To reduce Z0

– Reduce inductance– Increase capacitance

Both are achieved by– Reduction of loop area between conductors (less inductance)– Placing conductors as close together as possible (greater capacitance)– Increase of conductors’ width (less inductance and greater capacitance,

with respect to another conductor)

(x)

(x)

o

oo

IV

CL = Z

46


Defining Capacitor Usage

Capacitors are used for one of three primary functions.

Bulk Used to maintain constant DC voltage and current levels on a global basis due to IR drops within the power distribution network, and to recharge the distribution network (i.e., planes) cause by dI/dt consumption from components (typically 1-100 F).

Bypassing Diverts or steers RF currents from one location to another. Shunts unwanted common-mode RF currents from components or cables from entering susceptible areas in addition to providing other functions of filtering (bandwidth limited).

Decoupling Provides a localized source of DC power, and are particularly useful in reducing peak current surges propagated across the board. Prevents RF currents from being injected into the power distribution network during digital component edge transitions.

• Switching transient capacitance (0.01 F) – Used to supply short-term energy demands of the silicon during switching states.

• Line charging capacitance (0.1 F) – Used to charge capacitive transmission lines as well as supplying current necessary for the driver.

47


Effective Range of Decoupling Systems and Target Impedance

(Artwork provided courtesy – Ansoft Corporation)

48


Capacitance of the power and ground planes is defined by:

whereCpp = capacitance of parallel plates (pF)r = dielectric constant of the board materialA = area between the parallel plates (square inches or cm)D = distance spacing between the plates (inches or cm)k = conversion constant (0.2249 for inches, 0.884 for cm)

DAkC r

pp

Calculating Power and Return Plane Capacitance

49

Self-resonant frequency Through-hole capacitors

0.001

0.01

0.1

1

10

100

1000

10000

1.00 10.00 100.00 1000.00

Impedance (Ohms)

Frequency (MHz)

100 pF

0.001 uF

0.01 uF

0.1 uF

Capacitors and Resonance

1/4 inch or 0.64 cm leads – 15 nH


50

Self Resonant Frequency - SMT Capacitors

0.001

0.01

0.1

1

10

100

1000

10000

1 10 100 1000

Impedance (Ohms)

100 pF

0.01 uF

0.1 uF

Frequency (MHz)

0.001 uF


51

Effects of Capacitors in Parallel – Different Values


52

Effects of Capacitors in Parallel – Same Capacitor Values

How to calculate number of capacitors with the same capacitive value in parallel:

(Plot provided courtesy AVX Corp.)22 1

nCnESL

nESRZ

nESRESR

nLL

CnC

total

otal

total


53

Power and Ground Plane Capacitance


54

Conflicting Rules for PCB Decoupling

List created by Todd Hubing


55


Interconnects and I/O

56


Partitioning

Functional SubsystemsA group of components along with their respective support circuitry performing acommon function.

Quiet AreasSections physically isolated from other functional areas to prevents noise sources from corrupting susceptible circuits in the quiet zone.

Internal Radiated Noise CouplingRadiated RF coupling may occur between different functional subsections. To prevent this coupling, a fence or shield barrier may be required. A fence is a metal barrier secured to the ground plane(s) at intervals appropriate for the highest frequencies anticipated (at /20 wavelength intervals) and tall enough in height to prevent direct line RF radiated field coupling.

57


Isolation (Moating)

58


59


Bridging

I/O

MoatConnection to chassis ground

Bridge in moat

Vcc

VccVcc

Vcc

Vcc

Vcc

Ferrite bead-on-lead for optional power, if required, over a separate moat for power plane. (dotted line).

Vcc connector

Location ofoptionalgrounds tochassis

60


Image Plane or Moat Violation

61


Solution to All EMC Problems

62

Documents

Introduction to Printed Circuit Board Design for EMC ...2014.psessymposium.org/sites/2014.psessymposium... · Signal Integrity Concerns It only takes one item listed below to cause